If you thought our atmosphere contained the lion’s share of earth’s carbon—in the form of carbon dioxide—think again. Arctic permafrost houses twice the amount of carbon found in the air. And as the permafrost thaws, which it has done more rapidly as the climate warms, microbes living in the soil can kick into action and produce greenhouse gases such as methane and carbon dioxide in the process. Microbiologist Janet Jansson explains what scientists are learning about these little-studied microbial communities and whether their activity could contribute to climate change.